Currently, a charge pump is often used as a voltage booster or a voltage multiplier to raise an input voltage supplied from a low voltage source to a working voltage having a relatively higher level, so as to supply the high-level working voltage to various driving circuits that require a higher voltage.
Please refer to FIG. 1 that illustrates a prior art Dickson charge pump. As shown, the Dickson charge pump includes four charging capacitors C1˜C4, an output capacitor Cout, and five source/drain coupled n-type metal-oxide-semiconductor field-effect (MOS) transistors T1˜T5. The Dickson charge pump has an input end and an output end, the voltage levels thereat are represented by Vin and Vout, respectively. The charging capacitors C1˜C4 are separately used to receive clock signals CK and XCK, so as to increase the voltage level Vin at the input end to the voltage level Vout at the output end. The output voltage level Vout can be represented by:
  Vout  =            ∑              i        -        1            5        ⁢          (              Vin        -                  Vt          ⁡                      (            Mi            )                              )      
Where, Vt(Mi) is a threshold voltage of the source/drain coupled n-type MOS transistors T1˜T5. However, when multiple stages of transistors are connected in series in the Dickson charge pump, the threshold voltage would increase to thereby reduce the voltage conversion efficiency as being influenced by the so-called body effect.
FIG. 2 illustrates another prior art charge pump. As shown, this prior art charge pump includes two charge transfer branches, namely, a first branch A and a second branch B. The first charge transfer branch A includes eight (8) transistors MN1˜MN4 and MP1˜MP4, and four charging capacitors C1-C4. The second charge transfer branch B includes eight (8) transistors MN5˜MN8 and MP5˜MP8, four charging capacitors C5˜C8, and an output capacitor Cout. The charging capacitors C1, C3, C6 and C8 receive clock signals CK and XCK that have a polarity reverse to that of the clock signals CK and XCK received by the charging capacitors C2, C4, C5 and C7. Therefore, the two charge transfer branches A and B can be considered as two independent and reverse-phase charge pump circuits. In the charge pump shown in FIG. 2, the capacitors C1-C8 are off-chip capacitors and therefore the charge pump could not be integrated on a chip. In the case source/drain coupling transistors are used to substitute for the off-chip transistors, the source/drain coupling transistors are subject to breakdown caused by an overly large gate voltage.